Learning Processing: A Beginner's Guide to Programming Images ...

88 Learning processing
Looking at the above examples, we can see that a for loop consists of three parts:
• Initialization —Here, a variable is declared and initialized for use within the body of the loop. This
variable is most often used inside the loop as a counter.
• Boolean Test —This is exactly the same as the boolean tests found in conditional statements and
while loops. It can be any expression that evaluates to true or false.
• Iteration Expression —The last element is an instruction that you want to happen with each loop
cycle. Note that the instruction is executed at the end of each cycle through the loop. (You can have
multiple iteration expressions, as well as variable initializations, but for the sake of simplicity we will
not worry about this now.)
START
WITH THIS
for (int i = 0; i < 10; i+ +) {
3 RUN THE CODE
}
fi g. 6.7
Increment/Decrement Operators
1
4
5
DO THIS
RETURN TO #2
2
TEST THIS
IF FALSE EXIT
#4 & #5 ARE “INVISIBLE”
In English, the above code means: repeat this code 10 times. Or to put it even more simply: count from
zero to nine!
To the machine, it means the following:
• Declare a variable i,
and set its initial value to 0.
• While i is less than 10, repeat this code.
• At the end of each iteration, add one to i .
Th e shortcut for adding or subtracting one from a variable is as follows:
x ++; is equivalent to: x � x � 1; meaning: “ increment x by 1 ” or
“ add 1 to the current value of x ”
x � � ; is equivalent to: x � x � 1;
We also have:
x � � 2; same as x � x � 2;
x * � 3; same as x � x * 3;
and so on.
A for loop can have its own variable just for the
purpose of counting. A variable not declared at
the top of the code is called a local variable.
We will explain and defi ne it shortly.